Gas generators are used to produce a variety of gases for various purposes. Examples include hydrogen, oxygen, carbon monoxide and carbon dioxide generators. Hydrogen generators are of particular interest because they can be used to provide hydrogen gas that is used as an active material in a fuel cell battery.
Interest in fuel cell batteries as power sources for portable electronic devices has grown. A fuel cell is an electrochemical cell that uses materials from outside the cell as the active materials for the positive and negative electrode. Because a fuel cell does not have to contain all of the active materials used to generate electricity, the fuel cell can be made with a small volume relative to the amount of electrical energy produced compared to other types of batteries.
In some types of hydrogen fuel cells, hydrogen is formed from a hydrogen-containing fuel supplied to the negative electrode side of the fuel cell. In other types of hydrogen fuel cells, hydrogen gas is supplied to the fuel cell from a source outside the fuel cell.
A fuel cell system can include a fuel cell battery, including one or more fuel cells (such as in a fuel cell stack), and a gas source, such as a gas tank or a gas generator. Gas generators that supply gas to a fuel cell can be an integral part of a fuel cell system, they can be removably coupled to the fuel cell system, or they can include replaceable components containing reactants. A removable gas generator can be replaced with another one when the gas producing reactants have been consumed. Removable gas generators can be disposable (intended for only a one-time use) or refillable (intended for use multiple times) to replace consumed reactant materials.
Hydrogen generators can produce hydrogen using a variety of reactants and a variety of methods for initiating the hydrogen generating reactants. Hydrogen gas can be evolved when a hydrogen containing material reacts.
Some hydrogen containing compounds can be heated to evolve hydrogen gas, such as by a thermal decomposition reaction. Such thermal decomposition reactions, such as those of hydrides, can be advantageous over hydrolysis reactions because the yield of hydrogen is greater, it does not produce steam vapor that may have to be removed from the hydrogen gas, and freezing is not a concern.
Gauging the remaining hydrogen containing compound (fuel) is an important consideration during the operation of a hydrogen generator. For example, it can be advantageous for the user of a hydrogen generator to have an indication of the relative quantity of fuel remaining, in order to be able to replenish the supply of hydrogen containing compound with no or minimal disruption of the supply of hydrogen gas to a hydrogen consuming device such as a hydrogen fuel cell.
Various methods of measuring the quantity of hydrogen containing material in a fuel supply are known. Examples are disclosed in U.S. Pat. Nos. 6,155,099; 7,117,732; 7,698,936; 7,642,742; 7,254,983; 7,674,541; and 8,268,028. Some methods use measurement of an electrical magnetic or electro-magnetic property of the hydrogen-containing material, requiring additional hardware to conduct an electric current through the material or create a magnetic field; some require moving a hardware component; and some require monitoring, computing and tracking input and/or output energy for individual fuel cartridges as they are used, either in the hydrogen generator or the fuel unit itself.
In view of the above, an object of the present invention is to provide a method that overcomes disadvantages of the prior art. Preferably a fuel level indicator will have one or more of the following features or advantages: suitable for use with a fuel that is heated to evolve hydrogen gas, adds no or minimal cost to replaceable fuel units, results in low added cost for the hydrogen generator, does not require writing data regarding replaceable fuel units to a digital memory, allows fuel gauging of fuel units that have been previously used and removed from the hydrogen generator, operates reliably in any hydrogen generator orientation, requires no electric current to be conducted through fuel units.